1. Field of the Invention
The present invention concerns a method of refining a steel with a high chromium content by decarburizing it to a very low carbon content. By the present method it is possible to produce a high chromium steel with a low carbon content and a low nitrogen content in a short period of time. The method is also suitable for producing a chromium-containing steel with a nitrogen content regulated into a certain range.
The chromium-containing steels to which the present method can be applied are those of chromium content of 5% or higher, and examples thereof are Ni-Cr base stainless steels and Cr-base stainless steels.
2. State of the Art
In the AOD process which is widely practiced as a decarburization method of chromium-containing steel, chromium is easily oxidized when decarburization proceeds and carbon content becomes low. Then, Ar to oxygen ratio in the gas blown into the molten steel is increased so as to decrease loss of chromium. When the carbon content in the molten steel becomes to a certain low level a reducing agent such as ferrosilicon is charged and only argon is blown into the molten steel to stir it so that the chromium oxides occurred in the molten steel during the refining process so far may be reduced and recovered. Thus, there is obtained a molten steel with the carbon content lowered to a predetermined level and the chromium content of the level prior to the refining.
However, in the lower carbon range decarburization rate becomes so low that it takes long time to achieve the desired carbon content, and moreover, oxidation of chromium tends to proceed. To suppress oxidation of chromium it is necessary to increase the proportion of argon gas in the blown gas. This causes increase in consumption of argon gas, and the process becomes ineconomical.
It is practiced to use N.sub.2 gas, which is also non-oxidizing, instead of argon gas. This could be applied only to the refining of some limited steels.
Whichever of argon gas or nitrogen gas is used as the non-oxidizing gas, it is helpful to utilize vacuum refining as the way to promote decarburization at the low carbon range. For example, in the method described in U.S. Pat. No. 4,174,212 for the purpose of refining a high chromium stainless steel to such a low carbon content as 0.03% or less decarburization under atmospheric pressure with oxygen is carried out to achieve a carbon content as low as 0.4-0.2%, and thereafter, stirring with the non-oxidizing gas is continued and blowing of O.sub.2 is interrupted, and the pressure above the molten metal bath is continuously lowered to about 10 Torr or less so that boiling of the molten metal may occur, and thus, the desired decarburization is achieved. The refining method disclosed in Japanese Patent Disclosure No. 61-136611 adopts a similar process. In the method decarburization using an AOD device is carried out under atmospheric pressure, and then a vacuum refining device is used to continue decarburization under a reduced pressure of 20 Torr.
Development and proposal of further improved methods were made. In one of such methods decarburization is carried out to a carbon level of about 0.2% under atmospheric pressure by blowing a mixed gas of non-oxidizing gas such as argon gas and oxygen, and then, under the condition of such a reduced pressure as 200 Torr blowing of a non-oxidizing gas such as argon gas is continued to lower the carbon concentration. Another method includes, in furtherance to the above process, addition of reducing agent at the above vacuum refining for the purpose of reducing all the chromium which was oxidized during the preceding steps and thus achieving simultaneous decarburization and reduction of chromium oxides.
Generally, in refining chromium-containing molten steel where the formed chromium oxides are reduced with a reducing agent such as ferrosilicon, the ration W.sub.o /W, wherein W.sub.o stands for practical addition amount of the reducing agent and W for a theoretical amount of reducing agent necessary for reducing all the chromium oxides, is referred to as "Si-addition index". If a chromium-containing molten steel of a carbon content "C.sub.o " (weight %) is refined under vacuum for the period "t" (minutes) to lower the carbon content to "C.sub.l ", the following relation is held: EQU C.sub.1 /C.sub.o e -Kc.t
wherein Kc is a constant referred to as "decarburization reaction volume coefficient", which is expressed by the formula below: EQU Kc=(l/t) ln (C.sub.1 /C.sub.o)
and indicates easiness of the decarburization reaction in the vacuum refining.
Also, if nitrogen content before the vacuum refining is expressed with "N.sub.o " (weight %) and that after vacuum refining for a period of "t" (minutes) with "N.sub.1 ", the value "K.sub.N " expressed by the formula: EQU K.sub.N =(l/t) ln (l/N.sub.1 -l/N.sub.0)
is referred to as "denitration reaction volume coefficient", which indicates easiness of denitration reaction in the vacuum refining.
We carried out the above described refining on a chromium-containing steel of nitrogen content 0.15 wt. % and chromium content 17.2 wt. % by, after adding reducing agent with various Si-addition indices, blowing argon under a vacuum of 200 Torr at a rate of 0.3 Nm.sup.3/ min.ton-steel for 10 minutes, and measured at various Si-addition indices oxygen contents, carbon contents and nitrogen contents in the produced molten steel.
The results are shown in FIG. 2 as the relation between the oxygen content and Si-addition index; and in FIG. 3 and FIG. 4 as the relations between the decarburization reaction volume coefficient and the Si-addition index, and between the denitration reaction volume coefficient and the Si-addition index. As seen from these Figures, oxygen content is the chromium-containing molten steel shows a particular behavior at a Si-addition index around 1.0, and the value of Kc changes from large to small. On the other hand, KN shows little changes until the Si-addition index reaches 1.0, but shows a tendency to increase thereafter.
Based on this knowledge, we made further research on the relation between timing of adding Si-based reducing agent and the amount of addition to the chromium-containing molten steel described above, and discovered the facts that, according to the method explained later, carbon content in the steel may be lowered to 0.01% or less and that nitrogen content may be lowered to 0.02% or so.
In the above described refining method, if argon gas is used as the non-oxidizing gas, the N-content in the molten steel could be lowered to about 0.02%. However, depending on the kind of steels a higher N-content may be sometimes rather preferable. In case where the N-content is to be regulated to a certain value in the range of 0.03 0 0.10%, blowing only argon gas may result in an unnecessarily low N-content and may necessitate nitration step later. This causes, as the result, dissipation of expensive argon gas.
In the method described above (U.S. Pat. No. 4,174,212 or Japanese Patent Disclosure No.61-136611) which is a combination of atmospheric refining and vacuum refining, supply of 02 is stopped at a relatively high C-content, and as the consequence, loss of Cr by oxidation is not significant. However, sudden application of vacuum causes generation of large amount of CO gas which may bring about danger of explosion. The danger may be lightened if vacuum suction is slowly carried out. On the other hand, however, much longer period is spent for the process and another problem occurs, i.e., the molten bath temperature decreases and reaction becomes slow. Such a lower operating pressure as 10 Torr or less causes vigorous splashing of the molten steel, and this may result in plugging of hoppers for charging alloying elements. Thus, it is practically not operable to add reducing agent for recovering Cr from oxides thereof at the same time as the final decarburization step. Cr-recovery can be done by addition of reducing agent after completion of the decarburization, bu the period for the refining is prolonged.